A trend in the electronics industry toward greater miniaturization of semiconductor devices and toward the integration of a greater number of circuit elements into such semiconductor devices has been supported by the development of highly sophisticated manufacturing equipment. Circuit elements having features with a line width of two microns (2.times.10.sup.-6 meters) or even less are now the state of the art.
Photolithographic apparatus such as non-contact projection printers feature a degree in optical precision which allows images of pattern features to be projected from photomasks onto photosensitized semiconductor substrates with a high degree of resolution. The precise resolution or sharpness of the projected images is accompanied by a depth of focus which is substantially reduced to a range of about 10 microns about a focal plane along the projection axis of a typical, state-of-the-art projection printer. In addition to such focal depth limitations, other problems are encountered in imaging processes using even the most up-to-date equipment. For example, such disadvantageous technical phenomenon as arcing between adjacent lines tends to destroy unprotected photomasks and a commercially self-limiting factor appears in a demand for relatively smaller quantities of the relatively more specialized circuit devices with a greater degree of integration.
To support a new flexibility in the market introduction of newly developed devices through a recently advanced technology of computer-aided circuit design, photomasks for low-volume, specialized circuit devices are used in projection printers interchangeably with photomasks for devices which are expected to see higher volume, long-term markets. While photomasks for low-volume orders are typically used without first becoming protected against accidental damage, the photomasks for a large volume product are advantageously protected, for example, by coverplates, before they go into use.
The use of coverplates in the protection of photomasks is known to offer advantages over other forms of protection, such as the use of protective films or pellicles. However, protecting photomasks with coverplates is also known to shift the image plane of the photomask pattern away from the plane of the photomask pattern. When photomasks are protected by typical pellicles, such a shift of the image plane away from the image plane of an unprotected photomask does not occur.
The referred-to shift in the image plane of a coverplate-protected photomask with respect to an unprotected photomask has been found to present production problems when production schedules require a product mix of devices formed from unprotected photomasks as well as coverplate-protected photomasks. Mounting surfaces which locate the photomasks in projection printers with respect to mounting surfaces for locating photosensitized surfaces of semiconductor wafers need to be adjusted with respect to each other each time a change is made between unprotected photomasks and those protected by coverplates.
A trained technician is capable of performing a readjustment of the photomask mounting surfaces of currently available projection printers into a different focal plane within a period of approximately four hours. A similar length of time is then required to return the adjustment of the mounting surfaces for the photomasks to accommodate the coverplate-protected production photomasks. As a result, typically a full workday of photolithographic production is lost in adjustment time alone, whenever the production requires that product defined on unprotected photomasks is interposed between runs of coverplate-protected photomasks. When coverplate-protected photomasks are accommodated by one or more projection printers which remain adjusted to accommodate the coverplate-protected photomasks, such dedicated projection printers eliminate time losses due to the referred-to adjustments to accommodate focal shifts. However, dedicating projection printers entirely to processing from particular photomask structures becomes inefficient when the projection printers become under-utilized because of the special fixed focus setting.